(1) Field of the Invention
The present invention relates to an optical wavelength tunable filter using an acousto-optic effect, and more specifically, relates to an optical wavelength tunable filter which weights the intensity of a surface acoustic wave (SAW) which interacts with light.
(2) Related Art
In the optical wavelength tunable filter (AOTF) using the acousto-optic effect, weighting of the SAW intensity may be performed in an interactive area between light and the SAW, in order to suppress sidelobe in the filter property. As a weighting method in the conventional AOTF, for example, one using a SAW directional coupler, an oblique SAW guide, or a curved SAW guide is known (for example, see Japanese Unexamined Patent Publication No. 2004-219589 and 8-211349).
FIG. 18 is a block diagram showing one example of the conventional AOTF using a SAW directional coupler. In this AOTF, an optical waveguide 102 is formed on a substrate 101 comprising for example LiNbO3, the SAW generated on the surface of the substrate 101 by an interdigital transducer (IDT) 103 propagates on a directional coupler 104 formed along two parallel arm portions of the optical waveguide 102, and the light propagating on the respective arm portions of the optical waveguide 102 interacts with the SAW propagating on the directional coupler 104, thereby obtaining a required filter property. The directional coupler 104 has a SAW waveguide 104A formed on the respective arm portions of the optical waveguide 102, and a SAW waveguide 104B formed with a predetermined gap from the SAW waveguide 104A, so that the SAW given to the SAW waveguide 104B from the IDT 103 moves to the SAW waveguide 104A due to the directional coupling and then returns to the SAW waveguide 103B again, thereby weighting of the SAW intensity in the interactive area between the light and the SAW, to realize suppression of sidelobe in the filter property.
FIG. 19 is a block diagram showing one example of the conventional AOTF using an oblique SAW guide. In this AOTF, a SAW waveguide 105 guiding the SAW generated on the surface of the substrate 101 by the IDT 103 along the optical waveguide 102, is formed in an oblique direction with respect to the longitudinal direction of the optical waveguide 102. By weighting a coupling coefficient between the light propagating on the respective arm portions of the optical waveguide 102, and the SAW propagating on the SAW waveguide 105, suppression of sidelobe in the filter property is realized.
FIG. 20 is a block diagram showing one example of the conventional AOTF using a curved SAW guide. In the AOTF, for example, a straight optical waveguide 112 is formed on a substrate 111, and the SAW generated on the surface of the substrate 111 by the IDT 113 propagates on a curved area (SAW guide) placed between a pair of SAW walls 114A and 114B. Since the light propagating on the optical waveguide 112 interacts with the SAW propagating on the curved SAW guide, a required filter property can be obtained. In the AOTF using such a curved SAW guide, the intensity of the SAW interacting with the light gradually increases in the propagation direction of the light and then gradually decreases after having reached the maximum value, thereby realizing suppression of sidelobe in the filter property.
However, in the conventional AOTF there are problems in; the wavelength dependence of the filter property generated because the coupling length of the light and the SAW depends on the wavelength of the SAW, the polarization dependence of the filter property generated due to the asymmetry of weighting with respect to TE/TM mode lights respectively propagating on the respective arm portions of an optical waveguide, or an increase in sidelobe generated due to nonuniformity in sound-velocity distribution.
Specifically, in the conventional AOTF using the directional coupler shown in FIG. 18, for example as shown in the upper part of FIG. 21, the intensity distribution of the SAW with respect to the propagation direction of the light (in the y-axis direction in FIG. 18) becomes different, in the case of selecting the light (a) on the short wavelength side and (b) on the long wavelength side by the AOTF, due to different wavelengths of the SAW. Accordingly, the filter property of the AOTF has wavelength dependence, as shown in the lower part of FIG. 21, with the sidelobe generated at the time of selecting the short wavelength increasing as compared with the sidelobe generated at the time of selecting the long wavelength.
In the conventional AOTF using the oblique SAW guide shown in FIG. 19, since a discrepancy occurs in weighting with respect to the respective arm portions (the TE waveguide and the TM waveguide) of the optical waveguide, then for example as shown in the upper part of FIG. 22, the SAW intensity distribution on the TE waveguide becomes different from the SAW intensity distribution on the TM waveguide. Therefore, the filter property of the AOTF has a polarization dependence resulting from the refractive index distribution of the optical waveguide, thereby causing a polarization dependence loss (PDL) as shown in the lower part of FIG. 22.
Further, in the conventional AOTF using the curved SAW guide shown in FIG. 20, the sound velocity in the SAW propagation mode changes corresponding to the width of the SAW guide, so as to have a sound-velocity distribution with respect to the propagation direction of light. Accordingly, the filter property of the AOTF becomes such that an increase in sidelobe level occurs due to the sound-velocity distribution, for example, as shown by the solid line in FIG. 23, as compared with a case in which the sound velocity in the SAW propagation mode is uniform as shown by the dotted line.